Conventionally, some diesel engines have selective reduction catalyst incorporated in an exhaust pipe through which exhaust gas flow, said catalyst having a feature of selectively reacting NOx with a reducing agent even in the presence of oxygen. A required amount of reducing agent is added upstream of the reduction catalyst and is reacted on the catalyst with NOx (nitrogen oxides) in the exhaust gas to thereby reduce a concentration of the discharged NOx.
Meanwhile, effectiveness of ammonia (NH3) used as a reducing agent for reduction and purification of NOx is well known in a field of industrial flue gas denitration, for example, in a plant. However, in a field of automobile where safety is hard to assure as to running with ammonia itself being loaded, researches have been made nowadays on use of nontoxic urea water as reducing agent.
More specifically, when the urea water is added to the exhaust gas upstream of the selective reduction catalyst, the urea water is thermally decomposed into ammonia and carbon dioxide gas according to the following equation to depurate NOx in the exhaust gas through reduction by the ammonia on the catalyst.(NH2)2CO+H2O→2NH3+CO2  [Equation 1]
For exhaust emission control of the diesel engine, mere removal of NOx in the exhaust gas is insufficient; particulates (particulate matter) in the exhaust gas must be captured through a particulate filter. This kind of particulate filter employed requires to be timely regenerated by burning off the particulates to prevent increase of exhaust resistance due to clogging.
To this end, it has been conceived to additionally arrange flow-through type oxidation catalyst in front of the particulate filter; with accumulation of the particulates becoming increased, fuel is added to the exhaust gas upstream of the oxidation catalyst to forcibly regenerate the particulate filter.
More specifically, the fuel is added to the exhaust gas upstream of the oxidation catalyst to bring about oxidation reaction of the added fuel (HC) during passing of the fuel through the oxidation catalyst. The exhaust gas elevated in temperature by the reaction heat flows into the particulate filter arranged just behind so that temperature of a catalyst floor of the particulate filter is elevated to burn off the particulates, thereby attaining the regeneration of the particulate filter.
As actual measures for carrying out the above-mentioned fuel addition, it has been generally conceived that main injection of the fuel near a compression upper dead center is followed by post injection at non-ignition timing after the compression upper dead center so as to add the fuel to the exhaust gas. For effective utilization of the added fuel in forced regeneration of the catalyst and in order to conduct oxidization treatment of the added fuel before substantial lowering in temperature of the exhaust gas, it has been conceived preferable to arrange the particulate filter upstream of the selective reduction catalyst (see, for example, Patent Literature 1 mentioned below).
[Patent Literature 1] JP 2005-42687A